1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to a system and method of calibrating lamps.
2. Description of the Related Art
Light emitting diodes (LEDs) are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output and environmental incentives such as the reduction of mercury. LEDs are semiconductor devices and are driven by direct current. The brightness (i.e. luminous intensity) of the LED approximately varies in direct proportion to the current flowing through the LED. Thus, increasing current supplied to an LED increases the intensity of the LED and decreasing current supplied to the LED dims the LED. Current can be modified by either directly reducing the direct current level to the white LEDs or by reducing the average current through duty cycle modulation.
LEDs have component-to-component variation. For example, for a particular current, the brightness of one LED compared to another LED can vary by an amount that is noticeable by a human. Additionally, when one or more LEDs are assembled into a lamp and multiple lamps are arranged in proximity to each other, the variation between LEDs in different lamps can be sufficient to allow a human to notice a difference in the brightness of one lamp to another.
FIG. 1A depicts a lamp calibration system 100. In general, lamp calibration system 100 allows the brightness of lamp 102 to be tested and, if desired, adjusted within a tolerance level. The tolerance level can be a specific value or a range of values. The lamp calibration system 100 includes a lamp 102 situated in proximity to a light meter 104. The lamp 102 connects via exemplary power terminals 106 and 108 to voltage source 110 that supplies an alternating current (AC) supply voltage VAC—SUPPLY to lamp 102. Each lamp 102 is calibrated so that the brightness of lamp 102 is within a predetermined brightness tolerance. The voltage source 110 is, for example, a public utility, and the AC supply voltage VAC—SUPPLY is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. Lamp 102 includes a power control circuit 112 that converts the supply voltage VAC—SUPPLY into a regulated link voltage VLINK and an output current iOUT. The link voltage is, for example, an approximately constant voltage having a regulated value between 200V and 400V. The power control circuit 112 includes a lamp driver 114. The lamp driver 114 is a switching power converter, such as a buck converter, boost converter, or a buck-boost converter. Lamp driver 114 includes a switch (not shown), and a duty cycle of the switch is controlled by a switch control signal CS0 generated by controller 116. An exemplary power control circuit is described with reference to FIGS. 1 and 2 of U.S. patent application Ser. No. 11/967,269, entitled Power Control System Using A Nonlinear Delta-Sigma Modulator With Nonlinear Power Conversion Process Modeling, filed on Dec. 31, 2007, inventor John L. Melanson, and assignee Cirrus Logic, Inc. U.S. patent application Ser. No. 11/967,269 is referred to herein as “Melanson I” and is hereby incorporated herein in its entirety.
FIG. 1B depicts lamp calibration system 150, which represents a physical embodiment of lamp calibration system 100. Lamp 124 represents an exemplary physical embodiment of lamp 102. To calibrate lamp 124, lamp 124 is physically placed in a test apparatus 126 and connected to voltage source 110. Power control circuit supplies the output current iOUT to light source 118 to cause each of one or more LEDs in light source 118 to illuminate. Light meter 104 detects the light 119 generated by light source 118 and displays an indication of the brightness of light source 118 on display 120. Power control circuit 112 includes a trim module 122 that can be adjusted to vary the brightness of lamp 124 so that the brightness of lamp 102 as indicated by light meter 104 is within the predetermined brightness tolerance.
Power control circuit 112 is connected to housing 128 via power wires 132 of lamp 124. To expose the trim module 122, lamp 124 is partially disassembled by disconnecting housing 128 from lamp cover 130. Exposing the trim module 122 allows access to the trim module 122 and allows adjustment of the trim module 122 to adjust the brightness of lamp 124. After adjustment, lamp 124 is reassembled.
Partially disassembling lamp 124, adjusting the trim module 122, and reassembling lamp 124 results in a time consuming calibration process that is generally not conducive to manufacturing lamps in commercial volumes at competitive prices. Additionally, some conventional lamps 102 have inaccessible power control circuits and, thus, are not calibrated. Thus, it is desirable to have a different manner of calibrating a lamp.